218 ENTU-PAKASITES AND HYGIENE. 



latter, effective organs of locomotion in order to be able 

 to get at their host; they need fully developed apparatus 

 of sense, which leads to the hunting up and finding out of 

 tlie right animal to provide the food. The only thing 

 which may in some way be characteristic of such animals 

 is the formation of the parts of the mouth which are es- 

 pecially adapted for the reception of liquid materials — 

 for sucking. Since, however, these liquids never lie open, 

 but must be taken from another living organism, we find 

 beside such sucking apparatus also mechanical appli- 

 ances to wound the host, to sting, to cut, to saw, to scrape. 



Comparing the permanent parasites with these, we 

 shall observe several important changes. Above all 

 other things, it is no longer necessary for them to leave 

 their host and to come back to it again in case of ' need; 

 they can dispense with the possession of apparatus for 

 locomotion and subtile organs of sense; the latter liave 

 become superfluous, at least at the moment when the par- 

 asite has reached its permanent residence. Very natur- 

 ally such organs, if still there, will soon sink down from 

 their level of perfection; they begin to degenerate, and 

 this so much the more, the more settled the para- 

 site is on its bearer. We know several parasites which 

 are indeed permanent, which do not leave their host any 

 more, but still move around on the surface or in the in- 

 terior of the same. They, still have organs of locomo- 

 tion, and may possess also organs of sense for the per- 

 ception of outside impressions. These are, however, 

 rather reduced and simple in comparison to those of the 

 free-living animals. 



Such reduction may go on and affect the nervous system. 

 First the organs of sense disappear, and later it will be 

 most difficult to discover any traces of nerves. At the 

 same time the segmentation of the body becomes indis- 

 tinct, its shape more simple and clumsy, and the muscles 

 dwindle in corresponding manner. The end is that all 



